From the notes to Arrian's Array against the Alans by Flavius Arrianus, an interesting document given in English translation as well as the original Greek:

The Greek term Keltos was confusingly used to designate a German, the Celts themselves being called Galatoi. The ethnic designations of the auxiliary troops generally do not reflect the actual origins of their soldiers. Though auxiliary regiments were ethnically rather homogenous at the time of their original formation, the Roman army did not take any trouble to continue recruitment of replacements from the same background.